Interference Provides Clarity: Direct Observation of 2D Materials at Fluid-Fluid Interfaces

In Operando Study of Charge Modulation in MoS2 Transistors by Excitonic Reflection Microscopy

Monolayers of transition metal dichalcogenides (2D TMDs) experience strong modulation of their optical properties when the charge density is varied. Indeed, the transition from carriers composed mostly of excitons at low electron density to a situation in which trions dominate at high density is accompanied by a significant evolution of both the refractive index and the extinction coefficient. Using optical interference reflection microscopy at the excitonic wavelength, this (n, κ)-q relationship can be exploited to directly image the electron density in operating TMD devices. In this work, we show how this technique, which we call XRM (excitonic reflection microscopy), can be used to study charge distribution in MoS2 field-effect transistors with subsecond throughput, in wide-field mode. Complete maps of the charge distribution in the transistor channel at any drain and gate bias polarization point (VDS, VGS) are obtained, at ∼3 orders of magnitude faster than with scanning probe techniques such as KPFM. We notably show how the advantages of XRM enable real-time mapping of bias-dependent charge inhomogeneities, the study of resistive delays in 2D polycrystalline networks, and the evaluation of the VDS vs VGS competition to control the charge distribution in active devices.

Subphase Exchange Cell for Studying Fluid-Fluid Interfaces with Optical Microscopy

A subphase exchange cell was designed to observe fluid-fluid interfaces with a conventional optical microscope while simultaneously changing the subphase chemistry. Materials including phospholipids, asphaltenes, and nanoparticles at fluid-fluid interfaces exhibit unique morphological changes as a function of the bulk-phase chemistry. These changes can affect their interfacial material properties and, ultimately, the emergent bulk material properties of the films, foams, and emulsions produced from such interfacial systems. In this work, we combine experiments, computational fluid dynamics simulations, and modeling to establish the operating parameters for a subphase exchange cell of this type to reach a desired concentration. We used the experimental setup to investigate changes to a graphene film during a common wet-etching transfer process. Observations reveal that capillary interactions can induce defects and deformations in the graphene film during the wet-etching process, an important finding that must be considered for any wet-etching transfer technique for 2D materials. More generally, conventional optical microscopy was shown to be able to image the dynamics of interfacial systems during a bulk-phase chemistry change. Potential applications for this equipment and technique include observing morphological dynamics of phospholipid film structure with subphase salinity, asphaltene film structure with subphase pH, and particle film synthesis with subphase chemistry.

Ion and water adsorption to graphene and graphene oxide surfaces

Graphene and graphene oxide (GO) are two particularly promising nanomaterials for a range of applications including energy storage, catalysis, and separations. Understanding the nanoscale interactions between ions and water near graphene and GO surfaces is critical for advancing our fundamental knowledge of these systems and downstream application success. This minireview highlights the necessity of using surface-specific experimental probes and computational techniques to fully characterize these interfaces, including the nanomaterial, surrounding water, and any adsorbed ions, if present. Key experimental and simulation studies considering water and ion structures near both graphene and GO are discussed. The major findings are: water forms 1-3 hydration layers near graphene; ions adsorb electrostatically to graphene under an applied potential; the chemical and physical properties of GO vary considerably depending on the synthesis route; and these variations influence water and ion adsorption to GO. Lastly, we offer outlooks and perspectives for these research areas.

Hoop compression driven instabilities in spontaneously formed multilayer graphene blisters over a polymeric substrate

The blistering of elastic membranes is prone to elastic-solid as well as substrate-based mechanical instabilities. The solid-based instabilities have been well-explored in the mechanically indented blisters of elastic membranes over the rigid/solid substrates, but an integrated study illustrating the underlying mechanism for the onset of solid as well as substrate-based instabilities in the spontaneous blistering of a 2D material is still lacking in the literature. In this article, an extensive experimental as well as analytical analysis of the spontaneous blister-formation in the multilayer graphene (MLG) flakes over a polymeric substrate is reported, which elucidates the involved mechanism and the governing parameters behind the development of elastic-solid as well as viscoelastic-substrate based instabilities. Herein, a 'blister-collapse model' is proposed, which infers that the suppression of the hoop compression, resulting from the phase-transition of the confined matter, plays a crucial role in the development of the instabilities. The ratio of blister-height to flake-thickness is a direct consequence of the taper-angle of the MLG blister and the thickness-dependent elasticity of the upper-bounding MLG flake, which shows a significant impact on the growth-dynamics of the viscous fingering pattern (viscoelastic-substrate based instability) under the MLG blister.

Application of ZnO Semiconductor Nanomaterial Ink in Packaging and Printing Design

In order to obtain a flat and clear packaging printing pattern, the author proposes a printing method based on ZnO semiconductor nanomaterial ink. The method uses zinc acetate dihydrate as raw material, ethylenediamine as a complexing agent, absolute ethanol as a solvent, and ethyl cellulose as an auxiliary agent to prepare particle-free ZnO functional ink. The ink was spin-coated on a glass substrate, cured at different temperatures on a heating plate for 30 min, and passed through an X-ray diffractometer, a field emission scanning electron microscope, an infrared spectrometer, a synchronous thermal analyzer, an ultraviolet-visible spectrophotometer, and a transmission electron microscopy were used to characterize the synthesized inks and the resulting films. Experiments show that the decomposition temperature of particle-free ZnO conductive ink is much lower than that of zinc acetate precursor; the film cured at 300°C for 30 min has a smooth surface, uniform particle size, good crystallinity, and transmittance of up to 80%. After inkjet printing on the PI flexible substrate, after curing at 300°C for 30 min, the pattern surface is smooth and clear, and the outline is clear. Conclusion. The printing method is based on ZnO semiconductor nanomaterial ink. It has good application prospects in packaging and printing design.

2D Colloids: Size- and Shape-Controlled 2D Materials at Fluid-Fluid Interfaces

Advances in synthesis of model 3D colloidal particles with exotic shapes and physical properties have enabled discovery of new 3D colloidal phases not observed in atomic systems, and simulations and quasi-2D studies suggest 2D colloidal systems have an even richer phase behavior. However, a model 2D (one-atom-thick) colloidal system has yet to be experimentally realized because of limitations in solution-phase exfoliation of 2D materials and other 2D particle fabrication technologies. Herein, we use a photolithography-based methodology to fabricate size- and shape-controlled monolayer graphene particles, and then transfer the particles to an air-water interface to study their dynamics and self-assembly in real-time using interference reflection microscopy. Results suggest the graphene particles behave as "hard" 2D colloidal particles, with entropy influencing the self-assembled structures. Additional evidence suggests the stability of the self-assembled structures manifests from the edge-to-edge van der Waals force between 2D particles. We also show graphene discs with diameters up to 50 μm exhibit significant Brownian motion under optical microscopy due to their low mass. This work establishes a facile methodology for creating model experimental systems of colloidal 2D materials, which will have a significant impact on our understanding of fundamental 2D physics. Finally, our results advance our understanding of how physical particle properties affect the interparticle interactions between monolayer 2D materials at fluid-fluid interfaces. This information can be used to guide the development of scalable synthesis techniques (e.g., solution-phase processing) to produce bulk suspensions of 2D materials with desired physical particle properties that can be used as building blocks for creating thin films with desired structures and properties via interfacial film assembly.

Adsorption of Single and Multiple Graphene-Oxide Nanoparticles at a Water-Vapor Interface

The adsorption of graphene-oxide (GO) nanoparticles at the interface between water and vapor was analyzed using all-atom molecular simulations for single and multiple particles. For a single GO particle, our results indicate that the adsorption energy does not scale linearly with the surface coverage of oxygen groups, unlike typically assumed for Janus colloids. Our results also show that the surface activity of the particle depends on the number of surface oxygen groups as well as on their distribution: for a given number of oxygen groups, a GO particle with a patched surface was found to be more surface active than a particle with evenly distributed groups. Then, to understand what sets the thickness of GO layers at interfaces, the adsorption energy of a test GO particle was measured in the presence of multiple GO particles already adsorbed at the interface. Our results indicate that in the case of high degree of oxidation, particle-particle interactions at the water-vapor interface hinder the adsorption of the test particle. In the case of a low degree of oxidation, however, clustering and stacking of GO particles dominate the adsorption behavior, and particle-particle interactions favor the adsorption of the test particle. These results highlight the complexity of multiple particle adsorption and the limitations of single-particle adsorption models when applied to GO at a relatively high surface concentration.

Capillary force on an 'inert' colloid: a physical analogy to dielectrophoresis

"Inert" colloids are μm-scale particles that create no distortion when trapped at a planar fluid-fluid interface. When placed in a curved interface, however, such colloids can create interfacial distortions of quadrupolar symmetry - so-called "induced capillary quadrupoles." The present work explores the analogy between capillary quadrupoles and electric dipoles, and the forces exerted on them by a symmetry-breaking gradient. In doing so, we weigh in on an outstanding debate as to whether a curvature gradient can induce a capillary force on an inert colloid. We argue that this force exists, for the opposite would imply that all dielectrophoretic forces vanish in two dimensions (2D). We justify our claim by solving 2D Laplace problems of electrostatics and capillary statics involving a single particle placed within a large circular shell with an imposed gradient. We show that the static boundary condition on the outer shell must be considered when applying the principle of virtual work to compute the force on the particle, as verified by a direct calculation of this force through integration of the particle stresses. Our investigation highlights some of the subtleties that emerge in virtual work calculations of capillary statics and electrostatics, thereby clarifying and extending previous results in the field. The broader implication of our results is that inert particles - including particles with planar, pinned contact lines and equilibrium contact angles - interact through interparticle capillary forces that scale quadratically with the deviatoric curvature of the host interface, contrary to recent claims made in the literature.

Deviation from Equilibrium Thermodynamics of an Asphaltene Model Compound during Compression-Expansion Experiments at Fluid-Fluid Interfaces

Asphaltenes play a crucial role in crude oil behavior, and model compounds are often used to capture, mimic, and predict certain interfacial properties. In previous works, sorption of an asphaltene model compound (C5PeC11) was studied using surface pressure isotherms, where a deviation from the expected thermodynamic behavior of the interface during decane-water and air-water compression experiments was observed but not explained. In this work, the interfacial behavior of C5PeC11 was assessed at the decane-water and the air-water interfaces using a multiscale approach that includes: compression-expansion experiments on rectangular and radial Langmuir troughs, dynamic interfacial stress relaxation, and fluorescence microscopy imaging. Connections between molecular and microscopic phenomena strongly suggest that the nonthermodynamic response can be explained through a dynamic effect whose origin lies in the predominance of intermolecular forces in C5PeC11 molecules over the mechanical compression force applied. When aggregation begins at the air-water interface, stable structures are formed, and the nonthermodynamic phenomenon is not observed in subsequent compressions. However, at the decane-water interface, the initial aggregation is not consolidated due to the effect of the oil phase on the free energy of the interface allowing the high reproducibility of the dynamic effect in subsequent compression cycles. These results highlight the need to probe interfacial systems at various length scales to adequately separate equilibrium thermodynamics from dynamic responses.

Probing Interfacial Structure and Dynamics of Model and Natural Asphaltenes at Fluid-Fluid Interfaces

Asphaltenes are largely responsible for crude oil interfacial behavior. Due to their complex molecular nature, studying connections between interfacial properties and molecular structure is challenging, and these connections remain unclear. Several groups have reported on the interfacial behavior of asphaltenes, but a unified picture of both interfacial dynamics and thermodynamics is still missing. We seek to establish connections between asphaltene interfacial morphology and interfacial dynamics by combining interfacial dilatational deformation with microscopic structural imaging analysis. Understanding the behavior of natural asphaltene samples is made difficult by the inherent molecular variability. Therefore, we have also studied the behavior of an asphaltene model compound to draw fundamental structure-property relationships. This work contains simultaneous interfacial deformation and microscopy in systems of natural and model asphaltenes at air-water and decane-water interfaces. How the dynamics of natural asphaltenes influences the morphological and thermodynamic state of the air-water and decane-water interfaces is discussed based on the deviations observed between isotropic and anisotropic deformations. Areas where model asphaltenes can help us to understand the behavior of natural asphaltenes are identified such as its high surface pressure activity and aggregation character. An aggregation mechanism for model and natural asphaltenes is proposed based on an observed relationship between microscopic and millimetric aggregates.